The present invention relates generally to a control system for a compressor. More specifically, the present invention relates to a capacity modulation system for a compressor that can automatically adjust the capacity of the compressor.
Frequently, motors for driving compressors in heating, ventilation and air conditioning (HVAC) systems are designed to operate from standard line (main) voltages and frequencies (e.g., 230 V, 60 Hz) that are available at the location where the HVAC system is being operated. The use of line voltages and frequencies results in the motor being limited to one operating speed that is based on the input frequency to the motor. The operation of the motor at one speed, in turn, results in the compressor being limited to a single output capacity. Furthermore, motors that require their own controller or electronic drive, e.g., switched reluctance motors, cannot be used for these HVAC systems, as such motors cannot operate directly from standard (main) voltages and frequencies.
One problem with the compressor being limited to a single output capacity is that the compressor, especially reciprocating compressors, can produce excess capacity at reduced outdoor ambient temperatures. The excess capacity produced by the compressor adversely affects any system incorporating the compressor during SEER (Seasonal Energy Efficiency Rating) testing and in subsequent operation of the system. One attempt to solve the excess capacity problem in a compressor is discussed in U.S. Pat. No. 6,663,358, wherein a valve internal to the compressor is adjusted in response to operating conditions to effect a change in the capacity of the compressor. However, this mechanical solution may not be able to efficiently and cost-effectively obtain the desired reduction in capacity.
Therefore, what is needed is a cost-effective, efficient and easily implemented system to electrically provide for reduced compressor capacity at reduced outdoor ambient temperatures.